Polymers, e.g., polyolefins, polyvinyl halides, polyesters, polyamides, nitrile polymers, styrenic polymers and acrylate polymers, and elastomeric materials such as butadiene rubber, polyisoprene etc., are inherently unstable and susceptible to thermal oxidative degradation. Thus, these polymers and elastomeric material often require stabilization during melt processing. Exemplary stabilizers include phenolic antioxidants, hindered amine light stabilizers, ultraviolet light absorbers, organophosphites, antioxidants, metal salts of fatty acids, hydrotalcites, metal oxides, epoxidized oils, hydroxylamines, amine oxides, lactones, and thiosynergists.
Organophosphites are used broadly in the stabilization of polyolefins as non-discoloring antioxidants during melt processing, fabrication, and long term applications. Stabilization strategy of various polyethylene resins depends on the type (HDPE, LDPE, LLDPE, etc.), manufacturing process (gas-phase, slurry, solution), and catalyst (Ziegler-Natta, Chromium, metallocene, etc.) employed in the polymer production. Often times, the molar ratio of phosphite to hindered phenolics and the neutralizer package is dependent on the polymer grade. It is a common commercial practice to use combinations of sterically hindered phenols and phosphites in various molar ratios as a stabilizer system for polyethylene. Commonly used sterically hindered phenols include tetrakismethylene (3,5-di-t-butyl-4-hydroxylhydrocinnamate)methane, octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, bis(octadecyl)hydroxylamine, 3,5-bis(1,1-dimethylethyl)-4-hydroxy-benzenepropanoic acid and 2,6-di-t-butyl-4-ethyl-phenol. Commonly used phosphites include tris-nonylphenyl phosphite (TNPP) and tris(2,4-di-t-butylphenyl)phosphite, commercially sold under the trade names Alkanox™ 240 (Chemtura Corporation, Middlebury, Conn., USA), Irgafos™ 168 (Ciba Specialty Chemicals Corporation, Tarrytown, N.Y., USA), or Doverphos™ S-480 (Dover Chemical Corp, Dover, Ohio, USA).
TNPP and tris(2,4-di-t-butylphenyl)phosphite are commonly used in conjunction with octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate for melt stabilization of polyethylene. However, combinations of phosphites with octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate exhibit poor gas fading and high gel content when incorporated in polyethylene resins. Poor gas fading and high gel content render these stabilizers unsuitable for film applications. Without being bound to theory, it is believed that gels are small regions of high molecular weight polymers or loosely crosslinked polymers formed in the reactor and/or extruder, and are difficult to remove once formed. The gels are a common problem for low density polyethylenes and polyvinyl chlorides, and may cause distortions in film applications. Some prior attempts have been made to reduce gel content by adding anti-gel agents, such as polyethylene glycols/oxides or ethoxylated linear alcohols, as described in U.S. Pat. No. 4,540,538.
Thus, the need exists for safe and effective stabilizers that can effectively stabilize polymer resins and compositions against degradation.